paddy-drying-systems.ppt
- 1. Paddy Drying Systems By: M Gummert J Rickman Agricultural Engineering Unit IRRI, Los Baños , Philippines
- 2. Content • Why drying • Drying and quality • Drying methods • Mechanical drying systems • Drying strategies
- 3. Why Drying? • Rice is harvested at high moisture contents >20% • Quality deterioration starts immediately after harvest • The wetter the grain the faster the loss of quality • Different MC for different purposes (see Table) Purpose Required MC Potential problems 2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss 8 - 12 months storage <= 13% Insect damage > 1 year storage <= 9 % Loss of viability Milling 14% Damaged grains, cracking
- 4. Improper drying, 1 • Heat build-up – from natural respiration – excellent growth conditions for molds and insects • Mold development – propagate diseases in the grain – may release toxins into the grain – proper drying and storage can reduce propagation of molds • Insect infestation – insects are always a problem in stored grain – at lower moisture content insect activities are lower – proper drying helps keeping insects at acceptable level (4 insects per kg)
- 5. Improper drying, 2 • Discoloration/Yellowing – heat build-up in the paddy grain before drying – drastically reduces the market value of rice • Loss of germination and vigor – active respiration depletes the nutrition reserves – molds and diseases can reduce the ability of the seed to germinate – the lower the MC at the beginning of storage, the longer the seed remains viable
- 6. Improper drying, 3 • Loss of freshness/odor development – Heat build up -> musty odor in rice. – Reduces the market value of rice. – If from mycotoxin-producing fungi rice might become unusable. • Reduced head rice yield – moisture adsorption of individual dry grains with moisture contents below 16% - fissuring mixing dry with wet grains Exposing dry grains to humid air – Fissures cause cracking in milling process -> reduced the head rice recovery.
- 7. Equilibrium moisture content (EMC) • Rice is hygroscopic • Equilibrium moisture content (EMC) – If grain is exposed to air for a longer time it will reach EMC – Dry grain will adsorb water from humid air – Wet grain will dry • Grain properties – Moisture Content, MC – Temperature • Air properties – Relative Humidity, RH – Temperature Purpose Required MC Potential problems 2 - 3 weeks storage 14 - 18% Molds, discoloration, respiration loss 8 - 12 months storage <= 13% Insect damage > 1 year storage <= 9 % Loss of viability Milling 14% Damaged grains, cracking
- 8. Drying methods Drying Traditional Methods (Sun drying) Mechanical drying Field Drying Panicle drying Mat drying Heated-Air Drying Low-Temperature Drying Batch dryer Re-circulating Batch Dryer Continuous Flow Dryer Flash Drying Aeration In/store drying Pavement drying
- 9. Field drying • Why field drying? – Waiting for the thresher – Manual threshing • How? – Spreading the crop in the field – Stacking/piling • Advantages – Can reduce MC by 1% per day • Disadvantages – Rapid quality deterioration – Shattering – Losses to bird and rodents when spread in the field – Heat build up and rapid quality deterioration in piles – Re-wetting from straw in piles It is impossible to produce good quality grains with field drying practices. Field drying should therefore be avoided.
- 10. Panicle drying • Traditional method – harvested with a small knife (ani ani) – paddy grains that are still attached to the panicles – for drying small amounts of paddy. – stored in farmers’ houses, e.g. under the roof for protection from rodents. • Problems – low capacity – grains inside the panicle dry slower than the grains that are exposed directly to the sun. • Tips – turning of the panicles improves the drying process
- 11. Mat drying • Paddy can be placed on nets, mats or plastic sheets (canvas). • Advantages – Most hygienic method. – less contamination with stones and other dirt – Easy collection in case of sudden rainfalls. – Easy mixing • Disadvantages of nets – danger of re-wetting of the bottom grains from soil moisture
- 12. Pavement drying • Better-off farmers, grain collectors, traders and millers use drying pavements – specifically constructed for drying – multi purposes (basketball court) • Advantages – high capacity / economics of scale – can be partially mechanized – tools for mixing and grain collection – larger mills often use two or four wheel tractors • Disadvantages – capital requirements for the pavement – pollution with stones and dirt
- 13. Sundrying and quality Layer thickness • Spread the grains in thin layers, ideally 2-4 cm. • Too thin layers -> heat up very quickly • Too thick result in a large moisture gradient Mixing interval • mixing the grain is the most important activity for maintaining good quality • Turn or stir the grain at least once per hour, better every 30 minutes to achieve uniform MC. 60 65 70 75 80 85 90 0 1 2 3 4 5 6 7 Mixing interval, h Recovery, % 5 5.25 5.5 5.75 6 6.25 6.5 Drying Time, h Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines Intital M.C. = 24% Final M.C. = 14% 74 76 78 80 82 84 86 88 90 0 1 2 3 4 5 6 7 Layer thickness, cm Recovery, % 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 Drying Time, h Head rice Milled rice Drying time Source: IRRI, (Gayanilo) Location: Philippines Intital M.C. = 24% Final M.C. = 14%
- 14. Tips for better sundrying • Management – Layer depth of 4cm – Mixing every 30 minutes – Monitor moisture content – Monitor temperature • Protection – Cover the grain when temperature rise above 50- 60ºC – Cover during rain. – Prevent contamination – keep animals off the grain Use tools to improve sundrying
- 15. Heated air drying Components of a dryer • Main components – Drying bin – Air distribution system – Fan – Air heater • Optional – Conveyors • Accessories – Moisture meter – Dust separator
- 16. Options for heated air drying Fixed bed batch dryer Re-circulating batch dryer Continuous flow drying plant Tempering Section Drying Section Dryer Tempering bins In Out Air Grain
- 17. Fixed bed batch dryer (1) Flat bed dryer • Key features – Batch Capacity: 1-10t – Drying time 6-8h – Approximate prices: US$ 1000- 2000 – Kerosene or rice hull fired • Advantages – Simple and affordable • Disadvantage – Labor intensive – Moisture gradient – Temperature control
- 18. Fixed bed batch dryer (2) Low cost batch dryer • Key features – Batch Capacity: 0.2-2t – Drying time: 1-2 days – Approximate prices: US$ 100-200 – Wood, coal or rice hull fired • Advantages – Simple and affordable – Very cheap – Uses local storage structures • Disadvantage – Labor intensive – Moisture gradient – Temperature control
- 19. Fixed bed batch dryer (3) Reversible air flow dryer • Key features – Same as flat bed dryer – Airflow is reversed after ½ to ¾ of drying time was completed • Advantages – Minimized moisture gradient – Less labor intensive since mixing is eliminated • Disadvantage – Additional cost
- 20. Re-circulating batch dryer • Key features – Batch Capacity: 4-12t – Drying time: 8 hours – Approximate prices: US$ 8,000-10,000 – Kerosene fired • Advantages – Automatic operation – Produces excellent quality – Little floor area • Disadvantage – Wear of conveying elements – Problems with very wet paddy
- 21. Mechanical drying Methods Drying air temp.: 43ºC Air velocity: 0.15-0.25 m/s Airflow rate per t grain: >0.7 m³/s Power requirement: 1.5-2.5kW/t grain Layer depth: < 40 cm Drying time: 6-12 h Initial MC: up to 30%+ Drying Zone 10 15 20 25 30 0 5 10 15 Drying time, h MC, % w.b. Top Middle Bottom Avg. Drying Zone Dry Grains Wet Grains 10 15 20 25 0 48 96 144 192 240 Drying Time, h MC, % w.b. Top Middle Bottom Heated-air drying Low-Temperature Drying Drying air temperature: Δ T = 0-6 ºK Air velocity: 0.1 m/s Airflow rate per t grain: >0.05-0.4 m³/s Power requirement: 0.05-0.15 kW/t grain Layer depth: < 2 m Drying time: days to weeks Initial MC: 18% ( 28%) Advantages: Simple management Fast drying Affordable Low level of integration Disadvantages: 3-4% moisture gradient in final product, requires mixing or reduced layer depth Reduction in milling yield Danger of killing seeds Advantages: Very energy efficient Bins can be filled at harvest rate Maintains grain quality optimally Drying in storage structures Disadvantages: Increased risk with poor power supplies Requires bulk handling system (high level of integration in postharvest system)
- 22. Other drying systems • First stage dryers – Fluidized bed dryer – Rotary drum dryer • Low-temperature dryer (often second stage dryer) – In-store dryer – Aeration facilities
- 23. Flash dryer • Principle – Grains are pre-dried quickly in a fluidized bed – As a first-stage dryer in a two-stage drying strategy • Key features – Batch Capacity: 4-12t – Drying time: 10-15 minutes – Air temperatures: 110-120°C – Air velocity: 2.3 m/s • Advantages – Very fast pre-drying – High capacity • Disadvantage – For pre-drying to 18% MC only – High energy requirement Fluidized bed dryer from Thailand
- 24. In-store dryer Drying Zone Dry Grains Wet Grains • Principle – Slow EMC based drying with ambient air or slightly pre-heated air • Key features – Batch Capacity: 1… x.000 tons – Drying time: 4 days to 2 weeks – Air temperatures: ambient, 3-6°C above ambient – Air velocity: 0.1 m/s • Advantages – Produces very high quality – Low energy requirement – Drying in storage bin • Disadvantage – High risk if MC is > 18% – Long drying time
- 25. Drying Strategies • Decentralized on-farm drying – Requires quality incentive – Low utilization of equipment – Training and technical support service • Centralized drying – Contractors (service providers) – Mills – Economics of scale • Two-stage drying – Ideal process to produce best quality – Two machines are needed for one operation – First stage dryer dries only to 18%
- 26. Thank you